In recent years, gas turbine manufacturers have become increasingly concerned with pollutant emissions. Of particular concern has been the emissions of nitrogen oxides (NO.sub.x) because such oxides are a precurser to air pollution.
It is known that NO.sub.x formation increases with increasing flame temperature and with increasing residence time. It is therefore theoretically possible to reduce NO.sub.x emissions by reducing the flame temperature and/or the time at which the reacting gases remain at the peak temperatures. In practice, however, this is difficult to achieve because of the turbulent diffusion flame characteristics of present day gas turbine combustors. In such combustors, the combustion takes place in a thin layer surrounding the evaporating liquid fuel droplets at a fuel/air equivalency ratio near unity regardless of the overall reaction zone equivalence ratio. Since this is the condition which results in the highest flame temperature, relatively large amounts of NO.sub.x are produced. As a result, the conventional single-stage single-fuel nozzle spray atomized combustors may not meet newly established emission standards no matter how lean the nominal reaction zone equivalence ratio.
It is known that the injection of significant amounts of water or steam can reduce NO.sub.x production so that the conventional combustors can meet the low NO.sub.x emission requirements. However, such injection also has many disadvantages including an increase in system complexity, an increase in operating costs due to the necessity for water treatment, and the degrading of other performance parameters.
Attempts to achieve a homogeneous lean reaction zone by externally prevaporizing and premixing fuel and air at lean equivalence ratios have only limited applicability. These designs have typically been used for clean, very volitale fuels such as gasoline, jet fuel, etc., for regenerative cycle (elevated combustor inlet temperature), and at reduced pressures (less than 10 atmospheres). Beyond the increase in complexity, a serious drawback to this appraoch is the danger of autoignition and flashback. At 10 atomspheres pressure, the residence time required for complete vaporization of distillate fuel and that for autoignition is nearly the same. See, e.g., ASME Preprint 77-GT-69.
The problem of realizing low NO.sub.x emissions develops further complexity when it is necessary to meet other combustion design criteria. Among such criteria are those of good ignition qualities, good crossfiring capability, stability over the entire load range, large turndown ratio, low traverse number, long life and the ability to operate safely.
Some of the factors which result in the formation of nitrogen oxides from fuel nitrogen and air nitrogen are known and efforts have been made to adapt various combustor operations in light of these factors. See, for example, U.S. Pat. Nos. 3,958,416, 3,958,413 and 3,946,553. The processes used heretofore, however, have either been not adaptable for use in a combustor for a stationary gas turbine or have been inadequate for the reasons set forth below.
It is the object of this invention to provide a new dual stage-dual mode combustion system for a gas turbine which will operate over the entire gas turbine cycle at flame temperatures which will substantially reduce pollutant emissions to acceptable levels using various gaseous and distillate fuel.